In the most recent attack, media reports said pirates seized a ship off the Nigerian coast Sunday, holding its crew to ransom. The Russian foreign ministry said there were three Ukrainians, two Indians, one Indonesian and one Russian on board.

A media report said the crew members were taken off the vessel, operated by a multinational energy company. The kidnappers demanded a 200 million naira ($1.27 million) ransom. There has been no comment from the Nigerian government.

In January, an international maritime watchdog said sea pirate activity in international waters had subsided to the lowest level since 2008.

The International Maritime Bureau (IMB), an arm of the International Chamber of Commerce, said a total of 297 vessels were attacked by pirates last year, down from 439 in 2011.

The IMB's Piracy Reporting Center also recorded 67 attempted attacks last year.

Pirates took a total of 585 people hostage last year. Six people were killed and 32 injured.

Somalia and the Gulf of Aden remain the world's most dangerous maritime routes, accounting for nearly a quarter of the world's piracy incidents.

Seventy-five ships were attacked there last year. This figure, however, is well below the 2011 figure of 237 attacks.

Pirate activity intensified off the coast of Nigeria, where 58 crew members were attacked and 207 were taken hostage in 2012.